1. Field of the Disclosure
Embodiments of the present disclosure relate generally to electrical generators, and more specifically to a system and method for extracting the main field energy from a generator to accelerate generator shutdown.
2. Background of Related Art
Generators are used to generate a majority of the power on current electric power grids. These generators, which can be alternators or permanent magnet generators, for example, are used to turn mechanical energy from internal combustion, wind, nuclear, and hydraulic sources, for example, into electrical energy. Modern generators can produce hundreds of megawatts each.
Modern wound rotor generators include a rotor and a stator, each with one or more electrical windings. A magnetic field on the spinning rotor, generated by a current passing through the rotor winding, induces an electrical current in the windings of the stator creating AC electricity. The output voltage of the generator is often controlled by varying the current flowing through the rotor winding and therefore the strength of the magnetic field which induces voltage on the stator windings. The current in the rotor winding may be supplied by direct electrical connection to the generator controller by, for example, slip rings, or may be transferred from the generator controller through an armature winding to the rotor through electromagnetic induction. One or more loads can be connected to, and powered by, the output of the stator. In some embodiments, the output of the stator can be rectified to produce DC current.
A problem occurs, however, when a fault, such as a short-circuit, occurs on the load side and the energy delivered into the fault must be limited. In this condition, the generator controller can turn off the exciter current, but the rotor magnetic field does not immediately fall to zero due to the stored magnetic energy and the relatively long L/R time constant.
This condition results in the field current “free-wheeling” through the components of the rotor which, in turn, causes the main armature to continue producing current. Unfortunately, this current is being provided to a short-circuit, compounding the problem. This makes it difficult to clear the fault with the load (i.e., because it is still energized) and can overheat both the rotor and the stator. If this condition persists long enough, there is a risk of fire and permanent damage can occur to the load, the distribution system, and the generator.
What is needed, therefore, is a generator that includes a system and method to quickly dissipate the main field of a generator upon shutdown. It is to such a system and method that embodiments of the present disclosure are primarily directed.